After getting really annoyed by IEs PNG handling, I started some research and stumbled upon a fairly recent blog entry that really has the solution to the whole issue. I tried some javascript stuff before, but this forced me to specify width and height for images (which is generally a pain in the ass, if you have a big pile of code).
More information can be found at http://komodomedia.com/blog/index.php/2007/11/05/css-png-image-fix-for-ie/
I came along the solution that needed the height and width too, and since I was dissatisfied with it, I dove into a bit of vbscripting.
The (quite elegant) solution is to be found at:
http://www.geek.nl/~martijn/png/